The present invention relates generally to fuel-fired, forced air heating furnaces and, in a preferred embodiment thereof, more particularly provides an induced draft, fuel-fired furnace having a specially designed compact, high efficiency heat exchanger incorporated therein.
The National Appliance Energy Conservation Act of 1987 requires that all forced air furnaces manufactured after Jan. 1, 1992, and having heating capacities between 45,000 Btuh and 400,000 Btuh, must have a minimum heating efficiency of 78% based upon Department of Energy test procedures For two primary reasons, each relating to conventional heat exchanger design, the majority of furnaces currently being manufactured do not meet this 78% minimum efficiency requirement.
First, until recently, most furnace efficiencies were rated based upon "indoor ratings", meaning that the heat losses through the furnace housing walls to the surrounding space were ignored, the implicit assumption being that the furnace was installed in an area within the conditioned space (such as a furnace closet or the like) so that the heat transferred outwardly through the furnace housing ultimately functioned to heat the conditioned space. Under the new efficiency rating scheme, however, furnace efficiencies will be penalized for heat transferred outwardly through the furnace housing to the surrounding space on the assumption that the furnace will be installed in an unheated area, such as an attic, even if the furnace will ultimately be installed within the conditioned space.
Gas-fired residential furnaces are typically provided with "clamshell" type heat exchangers through which the burner combustion products are flowed, and exteriorly across which the furnace supply air is forced on its way to the conditioned space served by the furnace. The conventional clamshell heat exchanger is positioned within the furnace housing and is normally constructed from two relatively large metal stampings edge-welded together to form the heat exchanger body through which the burner combustion products are flowed. In the typical upflow furnace, the clamshell heat exchanger body has a large expanse of vertically disposed side surface area which extends parallel to adjacent vertical side wall portions of the furnace housing. In a similar fashion, in horizontal flow furnaces the clamshell heat exchanger body has a large expanse of horizontally disposed side surface area which extends parallel to the adjacent horizontally extending side wall portion of the furnace housing.
Due to the large surface area of clamshell heat exchangers, and its orientation within the furnace housing, there is a correspondingly large (and undesirable) outward heat transfer from the heat exchanger through the furnace housing which represents a loss of available heat when the furnace is installed in an unheated space. This potential heat transfer from the heat exchanger through the furnace housing side walls to the adjacent space correspondingly diminishes the efficiency rating of the particular furnace, under the new efficiency rating formula, even when the furnace is not installed in an unheated space.
The second heat exchanger-related factor which undesirably reduces the overall heating efficiency rating of a furnace of this general type arises from the fact the the typical clamshell heat exchanger has a relatively low internal pressure drop. Accordingly, during an "off cycle" of the furnace, this "loose" heat exchanger design permits residual heat in the heat exchanger to rather rapidly escape through the exhaust vent system (due to the natural buoyancy of the hot combustion gas within the heat exchanger) instead of being more efficiently transferred to the heating supply air which continues to be forced across the heat exchanger for short periods after burner shutoff. Stated in another manner, in the typical clamshell type heat exchanger the retention time therein for combustion products after burner shut off is quite low, thereby significantly reducing the combustion product heat which could be usefully transferred to the continuing supply air flow being forced externally across the heat exchanger.
In addition to these heating efficiency problems, conventional clamshell type heat exchangers have a long "dwell period" (upon cold start up) during which condensation is formed on their interior surfaces and remains until the hot burner combustion products flowed internally through the heat exchanger evaporates such condensation. This dwell period, of course, is repeated each time the furnace is cycled. Because of these lengthy dwell periods (resulting from the large metal mass of the clamshell heat exchanger which must be re-heated each time the burners are energized), internal corrosion in clamshell heat exchangers tends to be undesirably accelerated.
In view of the foregoing, it is accordingly an object of the present invention to provide an improved heating efficiency furnace having incorporated therein a heat exchanger which eliminates or minimizes the above-mentioned and other problems, limitations and disadvantages typically associated with conventional clamshell type heat exchangers.